The metastable zero field magnetic states of elongated Permalloy islands on square lattices of different geometries are investigated by means of magnetic force microscopy and numerical calculations. Keeping the island shape L / w = 10 fixed while varying the interparticle distances a, it is demonstrated that the relative frequencies of observed magnetic configuration at remanence strongly depend on the packing density of an array and the magnetic history ͑i.e., in which direction the array has been saturated before measurement͒. From 16 possible arrangements, only two are experimentally observed. For small L / a, a mixture of configurations, i.e., a spin ice, is found in agreement with previous studies, while for large L / a, an onion state unexpectedly prevails independent of the direction of the previously applied field. The results are discussed within the framework of potential theory and general stray field interaction. The relative frequencies of metastable states as well as the absence of the true ground state are in good agreement with theory.

Dot-by-dot analysis of magnetization reversal in perpendicular patterned CoCrPt medium by using magnetic force microscopy

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Dipolar spin ice has attracted much attention because of its intriguing ground state ordering and elementary excitation properties. We present experimental realizations of magnetic dipolar spin ice on periodic lattices with honeycomb symmetry. We have analyzed in particular the evolution and distribution of excitations with magnetic charges ±3 per vertex as a function of magnetic field and the distance b between the dipoles ranging from b = 0.4 to 1.7 µm. In all the dipole patterns investigated, we observe a surprisingly high abundance of ±3 magnetic charges at coercivity in the descending and ascending branches of the magnetic hysteresis. At the same time, these ±3 vertices form a charge ordered state with large domains, resembling an ionic crystal. Monte Carlo simulations of the magnetization reversal confirm in the framework of a macrospin model an enhanced abundance of ±3 magnetic charges at the coercive field. But much better agreement is achieved by taking into account the micromagnetic reversal mechanism, which proceeds via nucleation and domain wall propagation for dipoles aligned with the field and via coherent rotation for all others.

Artificial magnetic dipole arrays arranged on a square lattice exhibit a fascinating variety and complexity of configurations. Among the 16 possible configurations, six fulfill the spin ice rule of two dipoles pointing into a vertex and two point out. We present experimental realizations of magnetic dipole arrays and discuss the remanent state as well as the magnetization reversal in an external field.